Work holding means



Feb. 16, 1965 w. cuLL 3,169,777

woRK HOLDING MEANS Sheets-Sheet 2 United States Patent() 3,169,777 WORK HOLDING lVIEANS William Cull, Sutton Coldfield, England, assignor to Birlield Engineering Limited, London, England, a cornpany of Great Britain Filed Aug. 28, 1962, Ser. No. 219,955 Claims priority, application Great Britain, Aug. 29, 1961, 31,014/ 61 9 Ciaims. (Cl. 279-4) This invention relates to work holding means, and in particular to means for holding a workpiece by gripping ya peripheral surface of the latter during a machining operation.

Prior forms of Work holding devices such as taper collets, diaphragm chucks and the like suffer from the Ball chucks have been used comprising'balls which grip the outer surface of a workpiece and are moved inwardly for this purpose by a conical Wedging surface, but these ball chucks have suffered fromthe disadvantage that the reaction of the balls risborne by small point contact surface areas on the conical wedging surface, with the result that the latter becomes indented or brinnelled and the chuckbecomes inaccurate; The object of the invention is toprovide'work holding means ofthe ball chuck type which have a sustained accuracy forl relatively long periods of use.

According to the invention a work holding fixture' comprisesa grooved member arranged coaxially `with a ball cage member adapted to receive a workpiece, a ring of memberand retained inthe cage mem-ber so that they are accurately located axiallynfthe latter, the ballsbeing arranged to gripa surfaceofthe workpiece and the grooves so formed that the longitudinal axes thereof lie on a -conical surface coaxial with said members, and means to produce relative axial movement o-f said members so that the grooves engage the balls with a wedging action balls engaging the corresponding grooves in the grooved :40

to urge them away from the grooved member to the gripping position.

When intended to grip the outer surface of the workpiece the grooved member is internally grooved land forms an outer member arranged around the cage member, the balls being urged inwardly to the gripping position.

The engagement of each ball by a corresponding groove in the outer member produces a much lower surface loading spread over a greater area of the outer member for a given work-gripping force, as compared with the plain conical surface previously used in ball chucks. As a result, a fixture in accordance with the present invention enables the gripping force of the balls to be very much increased while the accuracy of the grooves is consistent over very long production runs. The grooves are preferably of elliptical cross-sectional shape, so that each ball has two areas of contact with the` corresponding the workpiece is centered by the cage member before the balls exert a gripping action.

Power-operated means are preferably provided to produce said relative axial movement of the outer and cage members, and these means may comprise a fluid-operated piston and cylinder assembly.

Both members and the piston and cylinder assembly may be mounted in an indexing head vwhich can be turned to index the workpiece for successive machining operations. Such an arrangement is conveniently used to -hold the outer member of a ball type constant velocity joint While the necessary ball tracks are machined in the outer member by means of an end-milling cutter.

The invention will nowv be further described with reference to the accompanying drawings which illustrate, by way of example, an indexing head incorporating a work holding fixture in accordance with the invention. In the drawings: j

FIGURE l is an axial sectional View of the indexing head,

FIGURE 2 is a detail of FIGURE l to a larger scale, and

FIGURE 3 is a detail sectional view on the line III- III in FIGURE l", also to a larger scale..

,An indexing sleeve 1 of the head is rotatably mounted in anti-friction bearings such asj2 in the body of van -associated machine (indicated generally -by 3) so that it can be indexed, and held in an indexed position, by means which form no part ofthe invention and hence will not be described herein'. 1 l' The indexing sleeve 1 isianged at 4 at `the front end and at the rear end Ahas bolted -to it by bolts such as 5 acylinder ofia piston 6 and cylinder 7 assembly/forming poweroperated means yof the fixture. vAn outerimember 8 of the iixture is bolted tothe iiange` 4 by bolts such as 9 and has a forwardly projecting nose `portion 10 in `which is machined an equiangularly arranged series of internal grooves suchas 12, shown in section in FIGURE a conical surfacevcoa'xial with the indexing sleeve 1.

An inner cage member `13a is a sliding it within the outer. member 8 ai`1d has a series of radial bores such as 14' in each of which a'ball 15 is located. All the .bores- 14 are located a common plane at right angles to the indexing axis, and each ball 15 engages in a corresponding one o f the grooves 12 in the 'outer member 8. The grooves 12 terminate at the front end of the nose portion 10 of the outer member 8 and a lip seal 16 l(see particularly FIGURE 2) is provided to prevent entry of dirt and chippings between the outer memberl 8 and the cage member 13. A flat annular spring 17 recessed into the axial bore 18 ofthe cage member 13 partly overlaps the radial bores 14 therein to retain the'balls 15 and prevent them falling inwardly.

A draw sleeve 19 connected to the rear or inner end of the cage member 13 is screwed at the rear end into the piston 6 of the piston and cylinder assembly, and a ring of return springs such as 20 urge the piston 6 and hence through the sleeve 19 the cage member 13 `forwardly to the free or non-gripping position. The'bore 18 of the cage member 13 has just suliicient clearance with -thereof are machined. The member has a cup-shaped end section which is received in the bore 18 and a stem section which is receivedinlocating bushes 24 wl`i ich lact to align themernber withinV the fixture. lThe ,bushes 24 can be changed to suit diffe-rent coupling members andare mounted in the iixed member 21. f To enable the cage Vmember vli'also Vto -be changed it is detachably connectedtothe[adjacent.endotthe draw sleeve ,19 by means of Va vclaw coupling formed by radial claws such as 25,and;26'onthesetwo parts. lOn iitting the outwardly `projectingclaws 26 pass between',

the inwardly projectingfclaws 25, after which relative rotation of the parts ymoves `oneset of claws behind the Yother to couple the two parts axially, the claws 26 Ybeing received withinan internal annular groove 2,7 ofthe cage member 13' and passing through slots inthe mem'her2/1. To clamp fthe workpiece 22 pressure fluidis admitted to the cylinder 7 viaa `running seal 2,8 `from ai VfixedV supply head 29 and acts on the forward `face ofthe pist-on -6 to move' the latter rearwardly, and this .draws the cage member 13 Vinto the outer member." Asta result the #gro'oves V12 in the outer member 8 engage the balls 1S with a wedging action so thattheygare n rgedradially inwardly into gripping engagement with the outer surface of theV workpiece 22.

Y s-,1593er fr oi latter, Vthe balls being arranged to grip a Vsurface `ini-,the workpiece kand` the grooves so formed that the longitudinal axes thereof lie on aconical surface coaxial with said members, and means to produce relative axial movement of said members so that the grooves engage the -balls with a wedging action to urgegthem radially into gripping contact with -t-hexworkpiece.

2. A work holding iixture according to claim 1, where- -inthe grooved member is internally. grooved and forms van outer4 member arranged around the cage member 4. A'fxture Vaccordingtov claim'd, wherein the `grooves Y f are of Velliptical cross-sectional shape. 'so-'that reac-h ball l An `'elliptical formation of lthe grooves 12 (see Vl'F-.G- f

URE 3) produces two spaced contact areas atA and .B

' Y between each ballk 15 land .the corresponding ygroove 12, the reaction to the grippingforce of the :ball 115 being shared ,equally between the two areas. kThese areas are .as showny respectively/positionedon the viiank'sVv of the,

:groove 12 inwardly ofljthe edge thereof and the sharing of` the 'load betweenthe two areas Aand B, kwhich are 'K :fg emselvefs'rofelliptical shape, enables a very high gripping -fvor'ce to be produced `without material yindentation-fofy v l:the surface of the groove over Vvery llong productionA runs. .lthas `:been found .thatthev optimum gripping eiri'ect isV #achieved if eacht-grooveyis inclined at anangle C (see FIGUREQ) of about 15 to the indexing "axis, VYand ,the lelliptical rformfof each groove is suchasito.producefaY l 1pfsufeang1e11 eHGUREn .fp'aspmximaelyw- Y The 'pressure angle as shown the angl'e'vsubtended at the center of each ball 15 bythe two areasfAand AB, of Y Vpressure contact of, that; ball with the l eorires/pending Y groove 12. 'Y l I claim: A 1. A work holding ltere comprisingaaroovedmember arranged coaxially'fwith ,a yhall'cage member adaptedl .to receive a'workpiece, a plurality oflballsengaging the' corresponding grooves in -the-groovedmember'and retained in the cage -member so that theyY are loca-ted axially Y has two Yareas ofcontact-with the.correszionding groove disposed Vrespectively on'the two Vflanks of the'latter'.

` 5. A fixture according to lclaim,4',`wl1er,ein the `pres- 'sureV anglesuhtended by the wovareas of contact at 'the center of the corresponding ball'is-'of the orderrof V60. y

6. A iixtur'eaccording to claim 4l, wherein thelongitudinal 4axis of Yeach groove ,is inclined at an `angleof the order of 15 .te the centralaxis .of theiixture. Y 7. A fixture according toY claim 1,r wherein power- Voperated'r'neans are provided to .produce said' relative axialV movement of the grooved and Vcage members, the power-,operated meansV comprising aiiiuid-operated piston andV ,cylindery assembly, 'thev pis-ton and cylinder of the assembly being respectively connectedto said members.

fixture accordingV tofclain'r 1, mounted in an in 'dexing head which'fcu beturned to index l the workpiece Y for successive Vmachining opeysrations.V K

9; A xture according to rclaim 8,' wherein the sleeves` to the outer ends lof:fwlciich,-tlie ,fixture Vmembers Vare respectivelyattached, the'indexingsleev beingaxiallyV VfixedV and `said means for producing relativeaxi'almove- K y indexing i head' comprises Ic'o'axially-arranged' indexing Aand,VV draw 

1. A WORK HOLDING FIXTURE COMPRISING A GROOVED MEMBER ARRANGED COAXIALLY WITH A BALL CAGE MEMBER ADAPTED TO RECEIVE A WORKPIECE, A PLURALITYOF BALLS ENGAGING THE CORRESPONDING GROOVES IN THE GROOVED MEMBER AND RETAINED IN THE CAGE MEMBER SO THAT THEY ARE LOCATED AXIALLY OF THE LATTER, THE BALLS BEING ARRANGED TO GRIP A SURFACE OF THE WORKPIECE AND THE GROOVES SO FORMED THAT THE LONGITUDINAL AXES THEREOF LIE ON A CONICAL SURFACE COAXIAL WITH SAID MEMBERS, AND MEANS TO PRODUCE RELATIVE AXIAL MOVEMENT OF SAID MEMBERS SO THAT THE GROOVES ENGAGE THE BALLS WITH A WEDGING ACTION TO URGE THEM RADIALLY INTO GRIPPING CONTACT WITH THE WORKPIECE.
 7. A FIXTURE ACCORDING TO CLAIM 1, WHEREIN POWEROPERATED MEANS ARE PROVIDED TO PRODUCE SAID RELATIVE AXIAL MOVEMENT OF THE GROOVED AND CAGE MEMBERS, THE POWER-OPERATED MEANS COMPRISING A FLUID-OPERATED PISTON AND CYLINDER ASSEMBLY, THE PISTON AND CYLINDER OF THE ASSEMBLY BEING RESPECTIVELY CONNECTED TO SAID MEMBERS. 